The present invention relates to a sheet feeder which is equipped with an image reading apparatus such as a copying machine or a scanner or an image forming apparatus such as a printer or a facsimile machine and which sequentially takes out sheets of document contained in a sheet container in a stacked manner to feed and carry the sheets to a carrier path. The present invention also relates to an image reading apparatus and an image forming apparatus that comprise this sheet feeder.
In recent years, an image forming apparatus or the like has been provided with an apparatus for reading an image of the sheet while automatically and sequentially taking and carrying the sheets of document one by one, so that the image is efficiently read and formed. On the other hand, due to progress of a digital technology, a speed of reading an image from the document, a speed of converting the read data into the electronic data and a speed of forming an image from the electronic data rise, and this makes it possible to process more documents at a high speed. Such a reading apparatus can contain a large number of sheets in a tray (a sheet container), namely, about 100 to 200 sheets. In addition, in accordance with progress of a document carrier of the image reading apparatus, various kinds of sheets can be carried.
Thus, if the number of contained sheets is increased, in the configuration such that the height of the tray is fixed, it is difficult to make a taking-out condition of each sheet from the tray even, so that an apparatus has been known, which enables to move up and down the tray by an elevating mechanism and maintains the uppermost face of the sheet contained in the tray at a predetermined height (see, for example, Japanese Patent Application Laid-Open No.11-199065 (1999), referred as a prior apparatus below). The prior apparatus intends to acquire an advantage of improving a convenience of the containing operation of the sheet by automatically moving down the tray when feeding of the sheet is initiated and the apparatus is brought to an emergency stop in midstream of carrier path due to a carriage error such as carrying jam or the like.
In the image reading apparatus for reading the image of the sheet while containing a large number of sheets such as documents in the tray and feeding and carrying them, there is a problem such that it takes a long time from the tray is moved up till the sheet can be fed to a carrier path so as to make a total reading time till reading of the sheet is completed longer when putting a tray having a large capacity for mounting the sheet on standby, moving up the tray by a signal with respect to the key operation or the like for initiating reading of the sheet, and initiating the operation for feeding the sheet to the carrier path for carrying the sheet to a reader.
In the prior apparatus, in the case of detecting occurrence of the carrying jam of the sheet, the tray is moved down to the lowest position. Accordingly, in the case of completing the jam handling and restarting the reading operation of the image of the sheet, it is necessary to move up the tray again. When the carrying jam is caused and the image is in the middle of being read or when no image has been read and the sheet should be returned to the tray while feeding the sheet from the tray again, the tray should be moved up again. However, when it is no need to return the sheet to the tray, there is no problem if the sheet is fed without moving down the tray. The prior apparatus, since the tray is always moved down to the lowest position when the carrying jam of the sheet occurs, the tray should be moved up to a height capable of feeding the sheet therefrom upon restarting feeding of the sheet for reading, and this involves a problem such that it takes a long time.